A position-measuring device, in the form of an angle-measuring device, includes a measuring standard and a scanning unit, which are able to be rotated relative to each other in that at least one of the two mentioned components of the position-measuring device (measuring standard and/or scanning unit) is rotationally mounted by a bearing. The rotational mounting usually serves the purpose of joining the rotationally mounted component, especially the measuring standard, to a shaft to be measured, e.g., of a machine tool, which rotates or is pivoted within a limited angular range when the machine tool is in operation.
The measuring standard, for example, is formed by a graduated reference disk, on which a measuring graduation extends in circular form, which is scannable by the scanning unit such that positional changes of the measuring standard with respect to the scanning unit are able to be detected.
The scanning of the measuring standard by the scanning unit may be carried out on the basis of various physical principles, for example. by optical, magnetic, or inductive scanning. When scanning the measuring standard, the scanning unit generates analog (electrical) output signals during a relative movement (rotational movement) of these two components with respect to each other, from which information about the position of the measuring standard in relation to the scanning unit is able to be derived. Toward this end, (digital) positional data obtained from the output signals are analyzed by a processing unit and by a possibly post-connected sequential electronics system. Depending on whether the position-measuring device is an incremental or an absolute position-measuring system, information about positional changes of the measuring standard in relation to the scanning unit, and/or information about the particular rotational position of the measuring standard in relation to the scanning device is able to be obtained in the processing unit by analyzing the positional data generated by the position-measuring device. In addition, information about the speed of the relative movement (rotary motion) and about changes in speed, etc., are obtainable by analyzing the temporal change of the position of the measuring standard in relation to the scanning unit.
For one, highly precise mounting of the rotatable component, usually the measuring standard, is of importance for the perfect functioning of the position-measuring device. To achieve this accuracy, it—or the associated shaft of the position-measuring system—is supported (without play, if possible) by a precision bearing, e.g., in the form of a precision ball bearing.
It is furthermore important that contamination of the measuring standard and the elements of the scanning unit used for scanning the measuring standard is avoided, if at all possible. To do so, the measuring standard and also the elements of the scanning unit required for its scanning may be situated inside a housing, which offers protection from dust and moisture. The housing must have a sufficiently tight configuration for this purpose. Nevertheless, when a position-measuring device is operated (possibly for years), it is rarely possible to completely avoid factors that have an adverse effect on the bearing quality, and also contamination.
Both effects impair the quality of the position measurement (angle measurement), but have completely different causes, e.g., wear of the bearing on the one hand, and the intrusion of foreign bodies into the housing of the position-measuring device on the other. Depending on which one of the two effects is responsible for the worsened position measurement, different countermeasures must therefore be taken, e.g., an exchange of the rotary bearing in the one case, and removal of the contamination and/or a repair of improperly sealed spots on the housing in the other.
Against this background, conventional devices and methods for monitoring bearing wear of a position-measuring device (angle-measuring device), as described, for example, in German Published Patent Application No. 10 2007 036 271 have the disadvantage that they are unable to reliably distinguish between the effects of bearing wear and the effects of contamination of the measuring standard or the scanning unit.